Fast pyrolysis is a rapidly developing second generation cellulosic bio-fuel production technology that holds extreme promise for the production of petroleum replacement fuels. The three products that are created by use of this technology are bio-oil, bio-char and syn-gas.
Several major problems exist with various forms of the technology, one of which is difficulty in obtaining rapid transfer of massive quantities of heat to biomass or other material in an oxygen and combustion free environment and in a cost competitive manner, i.e. low energy input and low capital cost input. The transformation from solid biomass to vapor to liquid at a specific controlled temperature must occur within a two to three second interval in order to be economical and viable. Some fast pyrolysis systems use direct heat transfer of heat to biomass or other material through direct surface contact with the reactor. This is problematic because the amount of surface area required to perform the reaction is very large thus requiring a larger reactor and making even application of heat difficult. Other systems use a moving heat carrier of either sand, steel shot, ceramic shot, or some form of catalyst. These systems use an open loop furnace whereby the heat carrier and char mixture is moved from the oxygen free environment to the furnace to be combusted, to remove the char from the heat carrier and to re-heat the heat carrier to be returned to the reactor to continue the process. The problem in these systems is that the oxygen free integrity of the system is compromised and the char cannot be harvested for sale as an additional value added product. Furthermore, precision control of the temperature of the heat carrier is not possible.
Bio-oil produced via fast pyrolysis is fast becoming a substitute for crude oil. However, the oil must first be upgraded by removing the oxygen, neutralizing the pH and removing the entrained char fines. Some fast pyrolysis systems attempt to remove the char fines by mechanical separation in the reactor, passing the vapor through a cyclone or passing the vapor through a ceramic baghouse before condensing in the condensers. The entrained char fines in the vapor destabilize the oil and thus must be removed. The cyclone system does not remove fine material thus allowing the char to stay in the vapor and is only good for removing larger particles or entrained heat carrier normally sand. The mechanical system removes a large portion of the char by volume, but, again not the fine char particles. The ceramic baghouse system causes the vapor to condense on the ceramic bags forming a hard crust and is therefore useless in operation. One attempt to address these problems has been a hot gas blow back filter developed by Pall Corporation that utilizes a porous metal or ceramic filter media which filters the vapor via char cake buildup on the porous metal or ceramic elements in a controlled method by differential pressure as applied from the input side of the filter. But, relatively high pressure applications used in fast pyrolysis systems are problematic because it is difficult to control the vapor flow rate through the condensers, and there are also problems with sealing of the oxygen free environment needed in order for the system to work.